High idle from cold start

[alan murphy]

Drive a 2000 reg megane classic 1.4 petrol, when I start it in the mornings it idles high +2000revs, over a 10min period it erratically, eventually returns to normal idle speed, this happens every morning or if it has been parked up for more than a couple of hours. I've changed the idle air control valve (think it's also called a stepper motor but could be wrong) and the air coolant temperature sensor, this has made no improvement at all.I don't think it's an air leak as it's only on a cold engine start that the problem is there. If anybody has any idea where to go from here, I'd appreciate it, thanks